1. Field of the Invention
This invention relates generally to a method and apparatus for removing noise from a sinusoidal signal, and more specifically to such a method and apparatus for removing a transient exponential noise component from a sinusoidal signal.
2. Description of the Prior Art
Electrical transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits that can cause collapse of the power system, serious and expensive apparatus damage, and personal injury. For instance, such a fault condition is caused by lightning-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a fault condition, line currents can increase to several times the normal value, thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line are isolated from the network by protective relays. The protective relays, by continuously monitoring ac voltages and currents, locate line faults and initiate line isolation via tripping of the appropriate circuit breakers. These faults are located by examining the relationship of the ac voltages and currents. Analysis of these ac voltages and currents is complicated by the presence of a decaying exponential component during the first few cycles after the fault. This decaying exponential is caused by the RL nature of the transmission line. The fault cannot be detected until the exponential component has decayed completely.
After the fault, utility personnel may inspect the transmission line to access the damage and make necessary repairs. The cost of inspections can be lowered significantly if the distance from the substation to the fault is accurately known. Also, repeated faults occurring at the same distance indicate the need to replace a weak insulator. A fault locator is used to determine the distance from the substation to the fault. Like the protective relays discussed above, operation and accuracy of the fault locator is hampered by the transient exponential component.
The offset compensator disclosed herein, when incorporated into a fault locator or a protective relay, removes the decaying exponential component from the sinusoidal fault current signal to provide a more accurate estimate of the distance to the fault or faster fault detection. Various prior art techniques are known for smoothing digital samples of analog signals. For example, the sampled values can be averaged over a predetermined time interval and this average used to modify sampled values within the interval. This technique removes the high-frequency components in the digital samples. The technique of the present invention is an improvement to the prior art smoothing technique in that it removes low-frequency components, e.g., a transient exponential noise component.